1. Field of the Invention
The present invention relates to liquid crystal display (LCDs), and more particularly to a method of restoring a damaged initial alignment of FLC material in the presence of an electric field via driving circuits used in normal driving operations of the LCD and a liquid crystal display using the same.
2. Description of the Related Art
Generally, LCDs display pictures by applying electric fields to a layer of liquid crystal material in response to an applied video signal, wherein the applied electric field controls the orientation liquid crystal molecules within the layer of liquid crystal material and therefore the light transmittance characteristics of the liquid crystal material. LCDs generally include an LCD panel having upper and lower glass substrates separated from each other by a layer of liquid crystal material, a light source (e.g., a back light unit) for emitting light to the liquid crystal display panel, a frame structure, a chassis for securing the LCD panel to the light source as one body, and a printed circuit board (PCB) for applying driving signals to the LCD panel.
LCD panels are generally fabricated by applying substrate cleaning, substrate patterning, substrate bonding, liquid crystal injecting, and drive circuit mounting processes. In the substrate cleaning process, detergent is used to remove impurities found on the surface of substrates used to form the LCD panel. The substrate patterning process includes an upper glass substrate patterning process and a lower glass substrate patterning process. The upper glass substrate of the LCD panel typically supports a color filter layer, a common electrode, a black matrix layer, etc. The lower glass substrate of LCD panel typically supports gate lines, data lines crossing the gate lines, thin film transistors (TFTs) formed at the crossings of the gate and data lines, and pixel electrodes formed in pixel region between the gate and data lines. The substrate bonding and liquid crystal injecting process include steps of spreading and rubbing an alignment film onto the upper and lower glass substrates of the LCD panel, adhering polarizer plates with polarization axes crossing perpendicularly to each other, bonding the upper glass substrate to the lower glass substrate using sealant material, injecting liquid crystal material through a liquid crystal injection hole, and sealing the liquid crystal injection hole. Upon performing the driving circuit mounting process, a lower glass substrate tape carrier package (TCP), on which integrated circuits (ICs) such as a gate drive IC and a data drive IC are mounted, is then connected to a pad part of the lower glass substrate in a tape-automated-bonding (TAB) process. Alternatively, the drive ICs can be mounted directly onto the lower glass substrate using a chip-on-glass (COG) system.
Following the aforementioned LCD panel fabrication processes, a module construction process is performed to fix the LCD panel, the light source, and the PCB together. Upon performing the module construction process, the PCB, the light source, and the LCD panel are sequentially arranged within a bottom cavity of a main frame. Subsequently, a top case is fixed to the main frame such that sides of the main frame and edges of the LCD panel are enclosed. In some cases, a bottom case may be fixed to the main frame such that it is arranged between the main frame and the top case. Accordingly, the bottom case can be used to enclose the bottom surface of the main frame. An input terminal of TCP is generally connected to an output pad of the PCB and an output stage of the TCP is connected to a signal wiring pad of the LCD panel. The light source includes a cold-cathode fluorescent lamp, a light guide panel, and optical sheets (e.g., a prism sheet, a diffusion plate, etc., sequentially arranged between a light guide panel and the LCD panel).
Generally, the liquid crystal material within LCDs exhibits an intermediate material phase between solid and liquid phases wherein liquid crystal molecules exhibit both fluidity and elasticity. Currently, the most common type of liquid crystal material used in LCDs include twisted nematic mode (TN mode) liquid crystal material.
Undesirably, TN mode liquid crystal material has a relatively low response speed and a relatively narrow viewing angle. To overcome the aforementioned problems, TN mode liquid crystal material can be replaced by ferroelectric liquid crystal (FLC) material having a response speed and viewing angle generally greater than that of TN mode liquid crystal material. FLC material exhibits a lamellar structure, wherein each layer of FLC material has the same electric and magnetic properties. Accordingly, when FLC material is driven, molecules of FLC material within the same layer spontaneously rotate (i.e., polarize) along a virtual cone in response to an applied electric field. In the absence of an applied electric field, molecules within the FLC material spontaneously polarize to an original alignment orientation. Accordingly, when external electric fields are applied to the FLC material, molecules within the FLC material rotate rapidly by interaction of an external field and exhibit spontaneous polarization. The response speed of FLC material is typically between a hundred or a thousand times faster than other modes of liquid crystal material. Further, FLC material has an inherent in-plane-switching property and can therefore be used to provide LCDs with wide viewing angles without special electrode structures or compensation films. Further, FLC material has an inherent in-plane-switching property and can therefore be used to provide LCDs with wide viewing angles without special electrode structures or compensation films. Depending on its behavior in the presence of applied electric fields, FLC material may be classified as V-Switching or Half V-Switching Modes.
V-Switching Mode FLC material exhibits the following thermodynamic phase transformations upon decreasing temperature: isotropic→smectic A phase (SA)→smectic X phase (Sm X*)→crystalline. At the isotropic phase, molecules within the FLC material are oriented and distributed substantially isotropically (e.g., randomly). At the smectic A phase (SA) phase, molecules within the FLC material are divided into symmetrically arranged layers of vertically arranged molecules. At the smectic X phase (Sm X*) phase, molecules within the FLC material are arranged according to an intermediate order between smectic A and crystalline phases.
FIG. 1 illustrates a graph of transmissivity of incident light versus voltage applied to a V-Switching Mode ferroelectric liquid crystal cell.
Referring to FIG. 1, the transmissivity of light incident to a V-Switching Mode FLC cell exhibiting the smectic X phase (Sm X*) is dependent upon the polarity of an applied driving data voltage (e.g., +V and −V). Accordingly, the arrangement of liquid crystal molecules within V-Switching Mode FLC material may be affected by the applied external voltage. V-Switching Mode FLC material beneficially has high response speed and wide viewing angle characteristics but disadvantageously requires a large amount of power in order to be driven because a capacitance value of the V-Switching Mode FLC material is relatively large. Therefore, a capacitance value of a storage capacitor used to maintain applied data voltages are also be large. Accordingly, if V-Switching Mode FLC material is used LCDs, an aperture ratio of the LCD becomes low since the power consumption of LCD and an electrode area of an auxiliary capacitor increases.
Half V-Switching Mode FLC material beneficially has a high response speed and wide viewing angle characteristics and further has a relatively low capacitance value. Therefore, Half V-Switching Mode FLC material is often used to display moving pictures.
FIG. 2 illustrates phase transformations of Half V-Switching Mode ferroelectric liquid crystal material.
Referring to FIG. 2, upon decreasing temperature below the phase transformation temperature (Tni), Half V-Switching Mode FLC material exhibits a phase transformation from the isotropic to the nematic phase (N*), below phase transformation temperature (Tsn), the Half V-Switching Mode FLC material exhibits a phase transformation from the nematic phase (N*) to the smectic C phase (Sm C*), and below phase transformation temperature (Tcs) the Half V-Switching Mode FLC material exhibits a phase transformation from the smectic C phase to the crystalline phase. Therefore, as the temperature decreases, Half V-Switching Mode FLC material exhibits the following thermodynamic phase transformations: isotropic→nematic (N*)→smectic C phase (Sm C*)→crystalline.
FIG. 3 illustrates the fabrication of a liquid crystal cell including Half V-Switching Mode FLC material.
Referring to FIG. 3, Half V-Switching Mode FLC material is typically injected into a liquid crystal cell at a temperature above Tni. Accordingly, upon being injected into the liquid crystal cell, molecules within the FLC material are oriented and distributed substantially isotropically (e.g., randomly). Upon lowering the temperature of the FLC material below Tni, molecules within the FLC material become aligned substantially parallel to each other along a direction dictated by the rubbing direction of an orientation layer and the FLC material exhibits the nematic phase (N*). If the temperature of the FLC material is further lowered the temperature below Tsn in the presence of an electric field, the FLC material exhibits the smectic phase (C*) and the liquid crystal molecules spontaneously polarize along the direction of the applied electric field to exhibit a monostable state, wherein liquid crystal molecules uniformly assume one of two possible molecular arrangements. If, on the other hand, the temperature of the FLC material is lowered below Tsn in the absence of the applied electric field, the liquid crystal molecules become separated into layers to exhibit a bistable state, wherein liquid crystal molecules within each layer uniformly assume one of the two possible molecular arrangements. Further, the distribution of the molecular arrangements within the layers is substantially random. In view of the above, it is generally more difficult to uniformly control the FLC material exhibiting the bistable than to uniformly control the FLC material exhibiting the monostable state. Accordingly, the Half V-Mode FLC cells are generally fabricated to exhibit the monostable state by cooling the FLC material below Tsn in the presence of an electric filed generated by applying a small direct current (DC) voltage to the LCD panel.
Referring still to FIG. 3, the symbol “{circle around (X)}” describes the direction of the applied electric field as extending out of the plane of the illustration. Therefore the spontaneous polarization direction of the FLC material also extends out of the plane of the illustration. Accordingly, electrodes used to generate the applied electric field are formed on upper and lower plates of the liquid crystal cell, extending out of the plane of the illustration. Further, the orientation layer described above is formed on the upper and lower plates.
The V-Switching Mode FLC material is aligned in the applied electric field after the aforementioned substrate bonding and liquid crystal injecting processes. Upon aligning the FLC material, data lines of the LCD panel are commonly connected to a first shorting bar, the small voltage is applied, a scan voltage greater than a threshold voltage of the TFTs is applied to gate lines commonly connected to a second shorting bar, and a common voltage (Vcom) is applied to a common electrode of the upper glass substrate. Accordingly, the common voltage (Vcom), from the common electrode, and the voltage, from the data lines, are applied to the FLC material.
FIGS. 4A and 4B illustrate the dependence of light transmissivity on a voltage applied to a Half V-Switching Mode FLC cell.
Referring to FIG. 4A, Half V-Switching Mode FLC cells containing FLC material aligned in the presence of an applied electric field generated by a voltage having a negative polarity (−V) (i.e., fabricated in the presence of an electric field having a negative polarity), transmit light in the presence of an applied voltage having a positive polarity (+V) by rotating a polarization axis of the light by 90°. The light transmissivity of the Half V-Switching Mode FLC cell increases proportionally to the intensity of an applied positive electric field generated by the positive voltage (+V). Further, the light transmissivity of the Half V-Switching Mode FLC cell attains a maximum value when the intensity of the applied positive electric field is greater than a fixed threshold value of the FLC material. In the presence of an applied voltage having a negative voltage (−V), the Half V-Switching Mode FLC cell does not rotate the polarization axis of the light. Accordingly, in the presence of an applied voltage having a negative polarity, the Half V-Switching Mode FLC cell transmits substantially no light (i.e., the Half V-Switching Mode FLC cell intercepts the light).
Referring to FIG. 4B, Half V-Switching Mode FLC cells containing FLC material aligned in the presence of an applied electric field generated by a voltage having a positive polarity (+V) (i.e., fabricated in the presence of an electric field having a positive polarity), transmit light in the presence of an applied voltage having a negative polarity (−V). Further, in the presence of an applied voltage having a positive polarity (+V), the Half V-Switching Mode FLC cell does not rotate the polarization axis of the light. Accordingly, in the presence of an applied voltage having a positive polarity, the Half V-Switching Mode FLC cell intercepts the light.
FIGS. 5A and 5B illustrate the orientation directions of Half V-Switching Mode FLC material in the presence of applied electric fields used to fabricate the liquid crystal cell and applied electric fields used to drive the liquid crystal cell, respectively.
Referring to FIG. 5A, when the Half V-Switching Mode FLC cell is fabricated in the presence of an externally applied electric field generated by a voltage having a negative polarity, the spontaneous polarization direction (Ps) of FLC material becomes uniformly aligned to the direction of the externally applied electric field having the negative polarity (E(−)). Referring to FIG. 5B, if, during a subsequent driving of the LCD panel, an electric field having a positive polarity (e.g., an electric field generated by applying a voltage having a positive polarity to the LCD panel) (E(+)) is applied to the fabricated Half V-Switching Mode FLC cell, the FLC material spontaneously polarizes along a spontaneous polarization direction (Ps) uniformly aligned with the direction of the applied electric field having the positive polarity. Accordingly, a polarization state of light incident to a lower plate of the LCD panel may be rotated to substantially align with the polarization direction of an upper polarizer on an upper plate via the FLC material, having the spontaneous polarization direction (Ps) uniformly aligned with the externally applied electric field having the positive polarity, and the incident light is transmitted through the upper plate. If, however, during driving of the LCD panel, the applied external electric field is generated by an applied voltage having a negative polarity (and thus itself has a negative polarity (E(−)), or if, during driving, no electric field is applied, the FLC material remains uniformly aligned along its initial spontaneous polarization direction (Ps) (characterized by the applied electric field having the negative polarity) and the incident light beam is not transmitted through the upper plate (i.e., the light is intercepted by the liquid crystal cell).
Because a cell gap (i.e., the distance between the upper and lower glass substrates of the liquid crystal cell) of related art FLC cells can be as narrow as about 1.2 μm, the alignment of the FLC material, generated in the presence of the applied electric field, may often be easily damaged by external physical impacts. More specifically, upon fabricating ferroelectric LCDs, an initial alignment is imparted to the FLC material after the aforementioned substrate bonding and liquid crystal injecting process. Accordingly, the initial alignment of the FLC material is likely to become damaged upon performing the aforementioned module construction processes, where physical impacts to the LCD panel frequently occur. In order to restore the damaged initial alignment of the ferroelectric FLC material, the TCP must be separated from the LCD panel and the voltage sources used to provide the initial electric-field-alignment must be re-connected to the signal wirings (e.g., the common electrode, the gate lines, and the data lines). The alignment restoration method described above, however, is excessively time consuming and can be extremely difficult to perform. Accordingly, an alignment restoration method that is capable of being more easily implemented is required.